The present invention relates to a xerographic recording apparatus for line-by-line exposure of the surface of a moving photoreceptor and, more particularly, to a circuit for minimizing registration errors in the sagital (slow scan) direction.
Image print bars used in xerographic recording systems are well known in the art. The print bar generally consists of a linear array of a plurality of discrete light emitting sources. Light emitting diode (LED) arrays are preferred for many recording applications. In order to achieve high resolution, a large number of light emitting diodes, or pixels, are arranged in a linear array and means are included for providing a relative movement between the linear array and the photoreceptor so as to produce a scanning movement of the linear array over the surface of the photoreceptor. Thus, the photoreceptor may be exposed to provide a desired image one line at a time as the LED array is advanced relative to the photoreceptor either continuously or in a stepping motion. Each LED in the linear array is used to expose a corresponding pixel in the photoreceptor to a value determined by image defining video data information.
For a print bar with a resolution of 300 sports per inch (300spi), a pixel size of 50.times.50 microns on 84.67 micron centers would be a typical configuration. In a xerographic application, where an 8.5 inch wide informational line is to be exposed, a linear array of approximately 2250 pixels, arrayed in a single row, would be required. (If two or more rows of parallel staggered rows of LEDs are used, the spacing between adjacent LEDs can be relaxed but the cost then increases.). One problem with prior art print bars is the difficulty of aligning all of the LED pixels in both the linear direction of the array and in the sagital plane, the sagital plane corresponding to the slow scan or process direction of motion of the photoreceptor. Present chip technology enables very accurate pixel placement in the linear direction, but individual pixels, or, more commonly, groups of pixels formed on the same chip, may be misaligned in the sagital direction, resulting in registration errors in later printed copies of the image being recorded. The significance of this type of registration error is amplified when a plurality of image bars are used, for example, in a full color printing system requiring accurate registration of simultaneous line exposures for each color.
It is known in the prior art to align LEDs in multiple rows in both the linear and sagital direction. U.S. Pat. Nos. 4,571,602 and 4,575,739, both to De Schamphelaere et al., disclose a method for correcting registration errors in an image projected onto the surface of a photoreceptor that results from unevenly positioned point sources along an LED array. In operation, driver control circuits 34 and 35 control the energization of individual LEDs located along first and second LED arrays 24 and 25, respectively. Delay registers in control circuit 34 delay the energization of individual LEDs in the first LED array relative to a photoreceptor speed signal and the energization of LEDs along the second LED array. This energization delay aligns each line of the image in the transverse direction.
U.S. Pat. No. 4,525,729 to Agulnek et al. discloses a method for simultaneously controlling at selected different time intervals the energization of individual LEDs within an LED array.
It is not disclosed in the known prior art how to identify individual pixels, or subarrays of pixels within a larger array, which are misaligned with respect to the other pixels and to correct for the misalignment or misregistration. According to the present invention, a circuit and method is provided for first initially calibrating a print bar to identify pixel to pixel mis-registration and then to provide appropriate circuitry for delaying drive signals which are sent to these misregistered pixels to delay their energization to insure that they are registered with the remainder of those LEDs which are in proper registration. More particularly, the present invention relates to an imaging apparatus for line-by-line exposure of the surface of a moving photoreceptor in a slow scan process direction by at least one linear print bar having a multiplicity of light emitting elements, at least some of said elements misregistered in the slow scan direction, the apparatus comprising: driver circuits for energizing said light emitting elements, circuit means for serially applying input data signal to said driver circuits during a line information time period, and
enabling circuit means for applying selectively delayed enabling signals to said drive circuit associated with a light emitting element, or group of light emitting elements, to be energized during the line information time so that the exposed line created by said light emitting elements is in linear registration.